As is known, in some types of machines (for example, internal-combustion engines for vehicle applications or for the generation of mechanical or electrical energy), of industrial plants and systems for the production of energy (for example, geothermal systems defined as “low-enthalpy systems” or for exploitation of the thermal energy produced by biomasses, which still present thermal flows that usually constitute waste but are potentially useful) the problem of recovering the thermal energy of a hot fluid at a relatively low temperature and of converting it into mechanical energy is posed.
For this purpose, it is known to use a Rankine cycle or Hirn cycle in which a working fluid in the liquid state is pressurized, heated via a heat exchange with the fluid from which the thermal energy is to be recovered up to total or partial vaporization, superheated or not, and then expanded in an expander that produces mechanical power available at its own output shaft (which can be exploited directly or converted into electrical energy via a generator driven by said shaft).
Given the low temperatures, the working fluid is generally constituted by an organic fluid, such as for example a chlorofluorocarbon in pure form or in mixture or a fluorocarbon, etc., in which case the cycle is usually referred to as ORC (Organic Rankine Cycle).
As expander, it is known to use a dynamic bladed expander or a volumetric expander, in this latter case, of the bladed type or some other type.
If the thermal power recovered from the working fluid is of limited intensity and temperature, there exist known technological and constructional difficulties regarding:
a) the production of a high-efficiency dynamic expander (turbine): the low flow rates of working fluid and the low enthalpies would lead to a general layout of the turbine (areas of passage, heights of blading, etc.) that prevents high (adiabatic, isoentropic) efficiency;
b) the contact between the hot working fluid and the surfaces of the machine causes a cooling of the working fluid and its condensation on the surfaces themselves, with loss of efficiency of conversion;
c) in the case of bladed volumetric expanders, the difficulties referred to in point a) cease to exist, even though the difficulties referred to in point b) remain, albeit to not such an important degree;
d) the frictions due to contact between the stator blades and the rotor blades, which are intensified in the presence of fluid vapours that expand, are the cause of a reduction of the efficiency of the machine; there derives therefrom the need for a technological improvement of the expanders with respect to the current state of the art.